A Barnes ’28 gpm’ pump is .465 cubic inch per rev in the small section, 1.395 in the large section, and 1.86 when combined. At 3450 rpm (assuming your motor speed times a two to one speed increase ratio), the pump will be about 26 gpm at combined flow, and 6.4 gpm on small section high pressure flow.
6.4 gpm at 3000 psi relief valve (that is BIG tonnage remember) is about 14 hp. That would be very intermittent peaks.
26 gpm at 500 to 750 psi is 10 to 14 hp. I would set the unloading at 500 psi and try it. If your particular working cycle and how fast you move wood is such that the motor has lots of cooling time, increase the setting to 550 or 600 psi. It will give you more pressure/force at the cylinder before the pump unloads to low speed. As long as the motor doesn’t overheat the windings, you can run more than 10 hp peaks.
26 gpm on 6 inch bore, assuming 24 inch stroke, is 6.6 seconds out. Assuming a 3 inch rod, it would be about 5 seconds to retract.
The 28 gpm two stage will be the simplest for you to work out the circuitry. I bought one on ebay for $35 because no one knew what it was, but you probably will spend closer to $350. The larger issue is that high flow makes the whole system quite spendy: you get out of the normal mass produced (thus cheaper) 15 gpm stuff into components that are harder to find. But it will be worth it. With that cylinder, you want lot of flow.
I don’t have time to get into line sizing but it is in the spreadsheet. Your suction and pressure lines to pump have to handle 26 gpm. The suction line should be at least 1.5 inches ID, preferably 2 inches. The pump may have a port smaller than what the pump mfr even recommends for line sizing. That is common in the hydr industry. Sigh…… Suction hose has no wire braid (doesn’t need as the pressure is low) but must have a large spiral wire reinforcement molded into the layers to resist collapsing under suction vacuum, especially at cold startup.
Suction strainer, if you use one, should be at least 40 gpm ‘rated’. Most mfr data is optimistic and at higher pressure drops and thinner fluids than I think realistic. If you use a strainer, I would go very large to minimize the pressure drops.
Pressure line from pump to the valve should be ¾ inch ID. 1 inch would be nice, but I would be content with ¾ ID.
Now it gets a bit trickier. With a 6 x 3 cylinder, the area of the closed side compared to the rod side is about 1.3 ratio. This means when 26 gpm goes into the closed side, 26/1.3 or 20 gpm comes out the rod side port, through the valve and back to tank. All well and good.
However, when retracting, 26 gpm into the rod side pushes 26 x 1.3 = 34 gpm out of the closed side port, through the valve, filter, and back to tank. So the control valve, the CL side hose, and the return lines and the filter have to be sized for at least 34 gpm, not 26 gpm.
That means preferably 1 inch ID hose from the cylinder to the valve. ¾ is workable, but velocity is high. ¾ and 1 inch pressure hose gets spendy in consumer channels.
Control valve has to be large enough for 26 gpm in, 35 gpm out, at a reasonable pressure drop. Most splitter valves that are ‘rated’ at 25 or 30 gpm will have quite a pressure drop. Pressure drop that does not do work burns fuel, and more importantly creates heat in the system that has to be dissipated by the tank or cooler. A pilot operated check valve can be used to send flow directly from cylinder to tank without going through the control valve. That works well with high flow circuits, but is overkill here.
Adaptors and fittings, JIC/SAE 37 degree flare or oring face seals. Adaptors oring boss. Get away from tapered pipe threads ASAP when you can.
Return line hose I would go up to 1-1/4 or 1-1/2. That can be fabric braid hose, or wire reinforced suction hose, so it is cheaper to go larger here than it is on pressure lines.
Return filter, a must, needs to handle 34 gpm at low enough pressure drops to still be filtering efficiently. You want the pressure drops low, and the line velocity low as the fluid goes into the tank. Sometimes a larger diffuser is used inside the tank to control the return flow.
